This is a pretty frequently asked question. You would have found the answer quicker by searching. How do I edit the "Sent from my BlackBerryÂ®..." message? - BlackBerryFAQ
That will only work for email. For texts, you cannot put an automatic signature.
What you can do is create and autotext entry. Go into Options > AutoText, press the menu key, and select new.
(insert signature here)
Then at the end of every text you could manually type zx and it would replace it with your signature.